An optical transmission system comprises an optical transmitter converting an electrical signal to be transmitted into optical form, (b) an optical fibre serving as a conductor for the optical signal, and (c) an optical receiver detecting the transmitted optical signal and converting it into electrical form.
A typical optical receiver comprises a photo detector connected to the input of a transimpedance amplifier. The photo detector converts the optical signal it has received into electric current that is supplied to the transimpedance amplifier. The latter generates at its output a voltage proportional to the incoming current, and thus a voltage proportional to the current of the photo detector is obtained from the output of the amplifier. The photo detector is usually either an avalanche photo diode (APD) or an optical PIN diode. Avalanche photo diodes are typically used at shorter wavelengths and optical PIN diodes at longer wavelengths, at which avalanche photo diodes generate a considerable quantity of noise. Transimpedance amplifiers are used for example for the reason that they allow comparatively good sensitivity properties to be achieved with a relatively simple construction.
Notwithstanding transimpedance amplifiers, one problem in optical receiver solutions lies in their poor dynamics: good sensitivity often entails a poor power tolerance and a good power tolerance again poor sensitivity. Poor dynamics for their part impair the operational flexibility of the receiver; for example when beginning to use a shorter fibre, an extra attenuator is necessary between the transmitter and the receiver.
Since the power level of the optical signal arriving at the receiver can in practice vary a great deal (depending on how long fibre is used), automatic gain control (AGC) is typically used in connection with the transimpedance amplifier to keep the amplifier's output voltage essentially at a constant value, when the incoming signal is higher than a predetermined threshold value.
There are in practice two different principles for widening the dynamic range of the receiver.
The first way of increasing dynamics is to adjust the resistance of the feedback resistor of the transimpedance amplifier on the basis of the amplitude of the incoming signal. The drawback of this method is that the adjustment generates large parasitics at the input of the amplifier, which will again impair the sensitivity of the receiver. The impairment is particularly marked when discrete components are used. When good sensitivity is aimed at, the stray capacitances on the input point of the amplifier are significant; even a small capacitance will impair the sensitivity of the receiver. Hence, it is essential that the parasitics on the input of the amplifier can be minimized.
Another way to widen the dynamic range is to use an adjustable resistive element in front of the amplifier. The resistance of the element is adjusted in response to the strength of the signal arriving at the amplifier in such a way that at higher levels the resistance is diminished, as a result of which the current coupled to the input of the amplifier will diminish (part of the current passes through the resistive element) and the amplifier is not saturated. This basic solution is known in several different variations. The main drawbacks of these solutions include complex implementation and stray capacitances generated by the requisite additional components, which will impair the sensitivity of the receiver.